cellular reproduction

cell division

no change in allele frequency

cell division

process by which cells make more cells

• Growth 

• Cell replacement 

• Healing 

• Reproduction 

• Copy genetic information 

• Separate copies 

• Cell division 

prokaryotes

• Eubacteria and archea 

• Binary fission 

  • DNA attaches to membrane 

  • DNA replication begins at a specific location and occurs bidirectionally 

  • The new circular DNA is attached to the cell membrane, close to the original circle 

  • The cell elongates symmetrically, separating the DNA attachment sites 

  • Cell division occurs as a new cell wall and membrane occurs at the midpoint 

bacteria genome organisation

• Highly expressed genes near origin 

• Many genes are on the leading strand (so that DNA polymerase and RNA polymerase do not clash 

• Genes with similar functions are clustered and co-expressed 

eukaryotes

• Replication of DNA more complicated 

• Cell cycle is regulated with check points 

• Chromosomes are found in homologous pairs 

chromosome structure

• Single = chromatid, double = sister chromatids 

• End = telomere (stable ends of chromosomes) 

  • Get shorter with each division 

• Centromere 

  • Spindle fibres attach to kinetochores 

interphase

lasts 10-14 hours

• G1 

  • Size and protein content increasing 

  • Proteins for DNA replication made (regulatory proteins) 

• S  

  • DNA replication 

  • Each chromosome duplicated (forms 2 sister chromatids, attached at a centromere) 

• G2 

  • Membranes break down 

• G0 

  • No preparation for division 

  • Specialised cells 

interphase events

• DNA replicated 

• Cell size increases 

• Organelles replicate 

mitosis

• Prophase 

  • Chromosomes condense (become visible) 

  • Centromeres radiate microtubules and migrate to opposite poles 

• Metaphase 

  • Prometaphase (microtubules of the spindle fibres attach to chromosomes, Nuclear envelope starts to break down) 

  • Metaphase (chromosomes align in the centre of the cell on the metaphase plate) 

• Anaphase 

  • Sister chromatids separate (become individual chromosomes and n.o. of chromosomes doubles) 

  • Travel to opposite poles 

• Telophase 

  • Nuclear envelope reforms 

  • Chromosomes decondense (become visible) 

cytokinesis

• Phragmoplast has a cell plate formed by vesicles, once it becomes large enough it fuses and cytokinesis is finished (plant) 

products of mitosis

• 2 genetically identical daughter cells 

• New cells are diploid 

• New cells have ½ cytoplasm and organelles 

meiosis

separation of haploid gametes

• Meiosis 1 = separation of homologous chromosome pairs (maternal and paternal) 

• Meiosis 2 = separation of sister chromatids 

Fertilisation = the fusion of haploid gametes 

Genetic variation = consequences of meiosis 

mitosis vs meiosis

• Both evolved from a single event (meiosis does not happen in every prokaryote) 

• Is same steps in all organism = single origin 

• Similar to mitosis (came from mitosis?) 

• Have no change in allele frequency (bar cytoplasmic genes and meiotic drive) 

meiosis 1

Prophase 1 

• DNA condenses 

• Homologous chromosomes condense and undergo synapsis (gene for gene pairing) 

• A bivalent forms once synapsis is complete 

  • Join at chiasma 

  • Shuffling of alleles 

  • Form recombinant chromatids 

Metaphase 1 

• Prometaphase (nuclear envelope breaks down, spindles attach to kinetochores) 

• Metaphase (homologous pairs line up in the centre of the cell, with bivalents randomly orientated) 

Anaphase 1 

• Homologous chromosomes separate (but sister chromatids do not) 

Telophase 1 and cytokinesis 

meiosis 2

Prophase 2 

• Chromosomes condense and nuclear envelope breaks down 

Metaphase 2 

• Spindles attach to kinetochores on chromosomes (prometaphase) 

• Chromosomes align in the centre of the cell 

Anaphase 2 

• Sister chromatids separate 

Telophase 2 and cytokinesis 

products of meiosis

• 4 cells produced from each original 

• Chromosome number halved 

  • Diploid -> haploid 

• Gametes are genetically different from each other and the parent cell 

sources of genetic variation

• Crossing over 

  • Segments of non-sister chromatids are exchanged during crossing over 

• Independent segregation 

  • Chromosomes are lined up randomly and at random orientations during metaphase 

  • Number of combinations = 2ⁿ 

• Gamete fusion 

  • Only one sperm and egg fertilise 

gametogenesis

• biological process of producing mature haploid sex cells (gametes)

• from diploid germ cells

• male = spermatogenesis

• female = oogenesis

spermatogenesis

• Spermatogonia are the initial pool of diploid cells that divide by mitosis 

  • A1 spermatogonia are used to replenish the pool of spermatogonia 

  • B spermatogonia eventually form mature sperm 

• The formation of sperm 

  • Replicate by mitosis multiple times 

  • Form identical diploid cells linked by cytoplasm bridges 

  • Known as primary spermatocytes 

  • Meiosis I produces 2 haploid cells (secondary spermatocytes) 

  • Meiosis II produces 4 haploid cells (spermatids) 

  • Cytoplasmic bridges break down and spermiation occurs (released into the seminiferous tube) 

  • Differentiate and remodel into mature spermatozoa as they travel along the tubules (spermiogenesis) 

cell cycle regulation

• Dysregulation = cancer 

• Cyclin 

  • Activate CDK (cyclin dependent enzymes) 

  • Target proteins that promote cell division (phosphorylate the) 

• Cyclin D-CDK and Cyclin E-CDK prepare the cell for DNA replication 

• Cyclin A-CDK helps initiate DNA synthesis 

• Cyclin B-CDK helps prepare the cell for mitosis 

• Checkpoints 

  • DNA damage checkpoint (before S phase) 

    • Is the DNA damaged? 

  • DNA replication checkpoint (at the end of G2) 

    • Is all the DNA replicated? 

  • Spindle assembly checkpoint (before anaphase) 

    • Are all chromosomes attached to the spindle? 

DNA damage checkpoint

• DNA damage activates protein kinases that phosphorylate p53 

• Phosphorylated p53 turns on genes that inhibit the cell cycle 

• Inhibiting the cell cycle gives time to repair the damaged DNA